Nicola J. Brassington

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time). The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption ‐ thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray sync hrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely t hat the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We describe a self-consistent model for the lobe dynamics and shock properties using the shell of thermal emission to the north of the lobe to estimate the lobe pressure. Based on this model, we estimate that the lobe is expanding to the southwest with a velocity of �2600 km s 1 , roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain γmax for the accelerated particles to be �10 8 at the strongest part of the shock, consistent with expectat ions from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strength s.

MODELS FOR LOW-MASS X-RAY BINARIES IN THE ELLIPTICAL GALAXIES NGC 3379 AND NGC 4278 : COMPARISON WITH OBSERVATIONS

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXB) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested

New Results on Particle Acceleration in the Centaurus A Jet and Counterjet from a Deep Chandra Observation

We present new deep Chandra observations of the Centaurus A jet, with a combined on-source exposure time of 719 ks. These data allow detailed X-ray spectral measurements to be made along the jet out to its disappearance at 4.5 kpc from the nucleus. We distinguish several regimes of high-energy particle acceleration; while the inner part of the jet is dominated by knots and has properties consistent with local particle acceleration at shocks, the particle acceleration in the outer 3.4 kpc of the jet is likely to be dominated by an unknown distributed acceleration mechanism. In addition to several compact counterjet features, we detect probable extended emission from a counterjet out to 2.0 kpc from the nucleus and argue that this implies that the diffuse acceleration process operates in the counterjet as well. A preliminary search for X-ray variability finds no jet knots with dramatic flux density variations, unlike the situation seen in M87.

GLOBAL STAR FORMATION RATES AND DUST EMISSION OVER THE GALAXY INTERACTION SEQUENCE

We measured and modeled spectral energy distributions (SEDs) in 28 bands from the ultraviolet to the far-infrared (FIR) for 31 interacting galaxies in 14 systems. The sample is drawn from the Spitzer Interacting Galaxy Survey, which probes a range of galaxy interaction parameters at multiple wavelengths with an emphasis on the infrared bands. The subset presented in this paper consists of all galaxies for which FIR Herschel SPIRE observations are publicly available. Our SEDs combine the Herschel photometry with multi-wavelength data from Spitzer, GALEX, Swift UVOT, and 2MASS. While the shapes of the SEDs are broadly similar across our sample, strongly interacting galaxies typically have more mid-infrared emission relative to their near-infrared and FIR emission than weakly or moderately interacting galaxies. We modeled the full SEDs to derive host galaxy star formation rates (SFRs), specific star formation rates (sSFRs), stellar masses, dust temperatures, dust luminosities, and dust masses. We find increases in the dust luminosity and mass, SFR, and cold (15-25 K) dust temperature as the interaction progresses from moderately to strongly interacting and between non-interacting and strongly interacting galaxies. We also find increases in the SFR between weakly and strongly interacting galaxies. In contrast, the sSFR remains unchanged across all the interaction stages. The ultraviolet photometry is crucial for constraining the age of the stellar population and the SFR, while dust mass is primarily determined by SPIRE photometry. The SFR derived from the SED modeling agrees well with rates estimated by proportionality relations that depend on infrared emission.

The X-ray spectra of the luminous LMXBs in NGC 3379: Field and globular cluster sources

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX> 1.2× 10 38 erg s -1 ). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-r ay binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters, and one is a confirmed field source. Due to the nature of the luminosity cut all sources are either neu tron star binaries emitting at or above the Eddington luminosity or black hole binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range

Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

We study the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low-luminosity limit and the comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of ∼8 × 10 35 erg s −1 , about ∼2–3 times deeper than previous investigations. We confirm the presence of the low-luminosity break of the overall LMXB XLF at log(LX) ≈ 37.2–37.6, below which the luminosity distribution follows a dN/d(ln L) ∼ const law. Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early-type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest an enhanced fraction of helium-accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the four times higher ionization temperature of He, such systems are subject to accretion disk instabilities at ≈20 times higher mass accretion rate and, therefore, are not observed as persistent sources at low luminosities.

Luminosity functions of LMXBs in different stellar environments

Aims. Based on the archival data from the Chandra observations of nearby galaxies, we study different sub populations of lowmass X-ray binaries (LMXBs) ‐ dynamically formed systems in globular clusters (GCs) and in the nucleus of M31 and (presumably primordial) X-ray binaries in the fields of galaxies. Our aim is to produce accurate luminosity distributions of X-ray bi naries in different environments, suitable for quantitative comparison with each other and with the output of population synthesis calculations. Methods. Our sample includes seven nearby galaxies (M31, Maffei 1, Centaurus A, M81, NGC 3379, NGC 4697, and NGC 4278) and the Milky Way, which together provide relatively uniform coverage down to the luminosity limit of 10 35 erg/s. In total we have detected 185 LMXBs associated with GCs, 35 X-ray sources in the nucleus of M31, and 998 field sources of which ∼ 365 are expected to be background AGN. We combine these data, taking special care to accurately account for X-ray and optical incompleteness corrections and the removal of the contamination from the cosmic X-ray background sources, to produce luminosity distributions of X-ray binaries in different environments to far greater accuracy than has been obtained previously. Results. We found that luminosity distributions of GC and field LMXBs d iffer throughout the entire luminosity range, the fraction of faint (log(LX) < 37) sources among the former being∼ 4 times less than in the field population. The X-ray luminosit y function (XLF) of sources in the nucleus of M31 is similar to that of GC sources at the faint end but differs at the bright end, with the M31 nucleus hosting significantly fewer bright sources. We discuss the p ossible origin and potential implications of these results .

Low-Mass X-ray Binaries and Globular Clusters in Centaurus A

We present results of Hubble Space Telescope and Chandra X-Ray Observatory observations of globular clusters (GCs) and low-mass X-ray binaries (LMXBs) in the central regions of Centaurus A. Out of 440 GC candidates, we find 41 host X-ray point sources that are most likely LMXBs. We fit King models to our GC candidates in order to measure their structural parameters. We find that GCs that host LMXBs are denser and more compact and have higher encounter rates and concentrations than the GC population as a whole. We show that the higher concentrations and masses are a consequence of the dependence of LMXB incidence on central density and size plus the general trend for denser GCs to have higher masses and concentrations. We conclude that neither concentration nor mass is a fundamental variable in determining the presence of LMXBs in GCs and that the more fundamental parameters relate to central density and size.

SIMULATED GALAXY INTERACTIONS AS PROBES OF MERGER SPECTRAL ENERGY DISTRIBUTIONS

We present the first systematic comparison of ultraviolet-millimeter spectral energy distributions (SEDs) of observed and simulated interacting galaxies. Our sample is drawn from the Spitzer Interacting Galaxy Survey and probes a range of galaxy interaction parameters. We use 31 galaxies in 14 systems which have been observed with Herschel, Spitzer, GALEX, and 2MASS. We create a suite of GADGET-3 hydrodynamic simulations of isolated and interacting galaxies with stellar masses comparable to those in our sample of interacting galaxies. Photometry for the simulated systems is then calculated with the SUNRISE radiative transfer code for comparison with the observed systems. For most of the observed systems, one or more of the simulated SEDs match reasonably well. The best matches recover the infrared luminosity and the star formation rate of the observed systems, and the more massive systems preferentially match SEDs from simulations of more massive galaxies. The most morphologically distorted systems in our sample are best matched to the simulated SEDs that are close to coalescence, while less evolved systems match well with the SEDs over a wide range of interaction stages, suggesting that an SED alone is insufficient for identifying the interaction stage except during the most active phases in strongly interacting systems. This result is supported by our finding that the SEDs calculated for simulated systems vary little over the interaction sequence.

Transient Low-mass X-ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration, and recurrence time. We apply this prescription to the population synthesis models of field LMXBs presented by Fragos et al., and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources. We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Finally, our comparison suggests that transient LMXBs are very rare in globular clusters (GCs), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations.